The invention primarily relates to a method for diagnosing the electrical condition of a spatially extended hardware component, in particular of a cable for transmitting electrical energy, in a spatially resolved manner by means of interference between pulses fed in into the component by a signal generator.
In addition, the invention comprises a device for diagnosing an electrical condition of a spatially extended hardware component, in particular of a cable for transmitting electrical energy, in a spatially resolved manner by means of interference between at least two pulses temporally offset by a pulse interval Δt that are fed into the component.
The current state of the art in the field of diagnosis of the condition of power supply cables is based upon conventional methods, for example, the measurement of partial discharges and determination of the loss factor (or tan (δ) measurement).
In addition, other methods are known, such as the relaxation current measurement method and the return voltage measurement method. In these measurement methods, the cable to be examined is first polarized with a DC voltage. Subsequently the depolarization current or return voltage following a short circuit is determined. These measurement variables, as in the case of loss factor determination, allow the condition of the dielectric to be assessed, for example, the moisture level, or the insulation resistance, etc. This is an integral measurement technique, however, one which does not allow a spatially resolved diagnosis of the cable condition. Furthermore, these methods are very sensitive and susceptible to interference from external environmental effects, such as humidity and temperature, with the result that they have not so far been widely used in the energy supply field.
In the partial discharge diagnosis (so-called “TE” diagnosis (=partial discharge diagnosis)) mentioned above, partial discharge defects are detected, assessed and localized. However, no sufficiently reliable conclusions can be drawn as to the cable condition and/or its remaining service life on the basis of a partial discharge diagnosis alone. This is only possible by using it in combination with other methods, for example the loss factor measurement mentioned above. A further requirement is that the voltage loading during the measurement be greater than the rated voltage, in order to obtain any reliable predictions as to the condition of the cable. On account of the high voltage loading, however, additional ageing and/or damage to the cable to be diagnosed can be caused. Spatial localization of a primary discharge site is possible, but the presence of multiple defects, long cable lengths with correspondingly high damping levels or moisture in the cable severely hampers any localization.
DE 10 2010 013 103 A1 discloses an apparatus and a method for the diagnosis of measurement objects using a measurement voltage, wherein the measurement method is essentially based upon determination of the loss factor.
The measurement takes place by applying a high voltage to the cable. Subsequently, measurement voltages are measured by means of a voltage detection arrangement and currents that are assigned to individual measurement objects are measured by means of the at least two current detection devices and analyzed in an analysis unit.
The high voltage required for the measurement can be generated by a large transformer, for example, but this is complicated and expensive. Alternatively the measurement high voltage can be produced by a resonance system or a so-called “VLF” method (=Very Low Frequency method), neither of which operate at the typical mains frequency of 50 Hz, however. Furthermore, these are examples of an integral measurement method, i.e., spatial resolution is impossible and consequently only the overall condition of a cable can be assessed.
JP 2001,153913 (A) further discloses a diagnosis method for the spatially resolved detection of a deterioration and/or degeneration in the electrical characteristics of a cable.
In this case, the frequency range reflectometry (“FDR method”=Frequency domain Reflectometry method) is used, which to an extent can also be applied for condition diagnosis. An advantage of this measurement method is that it can be carried out both non-destructively and with almost zero load, and consequently only requires comparatively low equipment complexity.
According to the method a resonant frequency analysis—or frequency spectral analysis is carried out, wherein the input signal is usually a frequency sweep. Impedance changes along a cable can therefore be determined in a similar manner to time domain reflectometry (“TDR”).
It is true that only gross changes in the electrical characteristics of the cable are detectable, which means that the electrical condition of the cable and/or the dielectric cannot be assessed with sufficient confidence. In some circumstances pre-polarization of the test sample with a high DC voltage allows defects to be more clearly exposed, allowing them to then be detected by carrying out a subsequent TDR measurement or FDR measurement.
In addition, U.S. Pat. No. 7,966,137 B2 discloses an analysis method or system based on resonance measurements on conductors (conductor resonance analysis).
This method is essentially an FDR measurement method with a resonance frequency analysis. This frequency spectral analysis delivers little in the way of reliable information about the condition of the cable, however, unless there are (reference) measurements available from an identical, intact cable.
US 2006/0097730 A1 further discloses an apparatus and a method for carrying out TDR reflection measurements.
In this case the JTFDR measurement method (JTFDR=“Joint Time Frequency Domain Reflectometry”) is applied, which represents a combination of the TDR and FDR measurement methods. In this method a matched input signal in the form of a frequency sweep with a Gaussian pulse as an envelope is fed into the cable to be examined and the characteristics of both methods are combined.
A disadvantage of this previously known method can be seen in the fact that, again, only changes in the impedance or the frequency spectrum respectively can be detected and analyzed.
US 2008/0048668 A1 furthermore discloses diagnosis methods for electrical cables using an axial tomography procedure.
In this arrangement the cable to be examined is connected in circuit at its beginning and end in a suitable manner, which can be effected for example by a variable voltage, an internal resistance, a variable frequency, a load impedance or the like, so that a standing wave is generated. By means of the voltage and current distributions and by measuring the apparent power at the input and output of the cable, balance equations can be set up, which by N-fold variation of the input parameters yield an equation system, with which all possible conductor parameters can be theoretically defined at any arbitrary point of the cable.
This method however implies a vast amount of measurement effort at both ends of the cable under test, because a large number of measurements with very many different input parameters must be carried out in order to obtain a result.